About this site

Why it exists

Looking up a tap drill should take five seconds. In practice you get a blurry chart image, a PDF behind a sign-up, or a page that gives you one number with no idea whether it suits the material you are actually cutting.

Tap Drill Size Chart is the chart as it should be: every thread on its own page, the drill in inches and millimetres, the percent of thread it really leaves, a larger drill for tough material, the form-tap hole, and the clearance hole for the bolt. Free, no sign-up, no paywall.

What is on the site

• 124 threads: UNC, UNF, UNEF, metric coarse and metric fine.
• 10 NPT tapered pipe sizes, 1/16 to 2 inch.
• 539 drills: number #1–#80, letter A–Z, fractional, and metric.
• A calculator that works in any material, for cut and form taps.
• Clearance holes to ASME B18.2.8 and ISO 273.

How the numbers are checked

The tap drills are published chart sizes. Nothing on this site is an engineering value invented for it. Every size lives in a data file, and the pages are generated from that file, so there is no number hand-typed into a template where it could drift.

On top of that, the build itself enforces a few rules:

• Every record is validated against a strict schema. A malformed or unexpected field fails the build rather than reaching a page.
• Every percent of thread is recomputed from that thread's own major diameter and pitch. If a stored figure and the recomputed one differ by more than 0.2, the tests report it by name rather than quietly adjusting either.
• Every drill a thread refers to must exist in the drill table, with a matching diameter.
• Where a standard genuinely does not cover a size, the page says so. It never substitutes a plausible-looking number.

What it is not

A specification. Published charts disagree with each other in places, tap makers each have their own recommendations, and no chart knows what your material, machine or tolerance actually are. For anything structural, pressure-bearing or safety-critical, check against your drawing and the chart that came with your tap. See the terms.

Corrections

If you find a number that looks wrong, please say so. Tell me the size, the field and what you think it should be, and point me at the source if you have one. The contact page has the address.